The present invention relates to adjustable diameter screw propellers and, more particularly, to propellers in which the diameter may be adjusted over a relatively large range and in which the pitch of the propeller increases as the diameter is decreased to compensate for loss of power of the propeller as its diameter is reduced.
It is known that a large ship, such as a tanker, bulk carrier or the like, can be operated more efficiently if driven by a relatively large diameter propeller (e.g., 20 to 40 feet or larger) turning at a relatively slow speed. The advantageous use of a large diameter propeller is described, for example, in "Large Diameter Propellers of Reduced Weight" by J. B. Hadler, R. P. Neilson, A. L. Rowan, R. D. Sedat, F. Seibold, and R. B. Zubal, Transactions of the Society of Naval Architects and Marine Engineers, 1982. The use of such a large diameter propeller, however, is normally not possible because the tips of the propeller blades would extend below the bottom of the ship's keel, thus creating a risk that the propeller and other parts of the ship's propulsion systems may be damaged if the ship is inadvertently grounded in shallow waters. Furthermore, the use of a large diameter propeller has the added disadvantage in that the ship is required to maintain a relatively deep draft in order to prevent the tips of the propeller blades from approaching too close to the water surface, since the blades of a larger propeller extend further in the vertical direction than those of a smaller propeller. If the tips of the propeller blades approach too close to the water surface or break the water surface, the cavitation or ventilation created by the propeller operating in such a manner results in losses that can offset the efficiency gained by using a large diameter propeller.
The latter disadvantage is of particular importance in a variable load vessel, such as a tanker, which operates a high percentage of the time in an unloaded or lightly loaded condition at minimum draft. Therefore, if a large diameter propeller is used in such a vessel, it is necessary to provide additional ballast or to trim the vessel to be lower at the stern than at the bow in order to insure that the propeller will be fully submerged when the vessel is unloaded or lightly loaded. However, the use of additional ballast or a greater trim at the stern increases the power needed to propel the otherwise unloaded or lightly loaded ship, thereby again offsetting the efficiency gained from using a large diameter propeller.
Recently, it has been recognized that the above-mentioned drawbacks of a large propeller are overcome by using a propeller whose diameter could be adjusted over a wide range, e.g., 20% to 50%, such that a ship could operate with a relatively large diameter propeller when fully loaded and in deep waters but could also reduce the size of its propeller when lightly loaded or in shallow waters.
Known configurations for adjustable diameter propellers typically include a hub, two or more propeller blades each mounted to pivot about a respective transverse pin in the hub and a mechanism within the hub for tilting each blade between a position in which each blade is substantially perpendicular to the axis of rotation of the hub and a position in which each blade is inclined towards the aft end of the hub.
Such known blade tilting configurations for adjustable diameter propellers have the shortcoming in that the mechanism for tilting the propeller blades is generally difficult to implement for large propellers, such as those which are 20 to 40 feet in diameter or larger. Furthermore, it is also difficult to reliably seal such a mechanism to prevent leakage of water from outside the hub.
In an adjustable diameter propeller, it is desirable to have the pitch of the propeller increase as the propeller diameter is decreased, since for a fixed pitch and rpm the power of a propeller decreases with decreasing diameter. Therefore, having the pitch increase with decreasing propeller diameter would compensate for such loss of power. Known blade tilting configurations for varying the propeller diameter have the additional shortcoming in that the pitch of the propeller generally does not change as the propeller blades are tilted towards the aft end of the hub.
Accordingly, a need clearly exists for a screw propeller construction which provides for adjustment of the propeller diameter over a relatively wide range and for increasing the pitch of the propeller as its diameter is decreased and which includes a mechanism for propeller diameter adjustment that is suitable for relatively large propellers and reliably sealed from water outside the hub and for retention of lubrication for the mechanisms within the hub.
Conventional adjustable pitch propellers, in which the propeller blades are attached to rotatable radial spindles in the hub, have blades which are in some instances skewed, in that the angle between the axis of the spindle and the radius line from the axis of the hub to the tip of the propeller blade attached to the spindle is greater than zero. The purpose of such skew, which is typically 25.degree. or less, on an adjustable pitch propeller is to avoid vibration of the propeller caused by each blade as it passes through the turbulence in the water behind the ship. The presence of such blade skew in a conventional adjustable pitch propeller results in an incidental reduction in the diameter of the propeller as the blades are rotated in a direction to increase the propeller pitch. However, for the degree of blade skew used in conventional adjustable pitch propellers, the extent of the change in propeller diameter is small, e.g., less than 5%, and the use of blade skew in such propellers is not intended to make the propeller diameter adjustable.